To weld automobile bodies and automobile components, resistance spot welding, mainly direct spot welding, has conventionally been used. Recently, however, indirect spot welding and the like have been used.
The characteristics of such direct spot welding and indirect spot welding are described below with reference to FIG. 1.
Those two forms of spot welding are alike in that at least two overlapping metal sheets are welded.
FIG. 1(A) illustrates the direct spot welding method. As illustrated, direct spot welding is a method that sandwiches two overlapping metal sheets 1 and 2 from above and below with a pair of electrodes 3 and 4 and applies current while applying pressure with the electrodes 3 and 4 to form a weld 5 using resistance heat generated between the metal sheets. The electrodes 3 and 4 are provided with force controllers 6 and 7 and a current controller 8, which constitute a mechanism that allows for control of the electrode force and the current.
The indirect spot welding method illustrated in FIG. 1(B) is a method in which a weld 25 is formed between two overlapping metal sheets 21 and 22 by holding an electrode 23 against one of the metal sheets 21 while applying pressure with the electrode 23, attaching a feeding point 24 to the other metal sheet 22 at a location remote from the electrode 23, and allowing current to flow between the electrode 23 and the feeding point 24.
Current control standards used by transport equipment manufacturers for welds formed by resistance spot welding often require that welds be oval nuggets formed after complete fusion between the metal sheets such as those obtained in direct spot welding. Therefore, between the above-described welding methods, the direct spot welding method is used when there is ample space for an opening allowing the metal sheets to be sandwiched from above and below.
In actual welding, however, there is often not enough room, and it is often difficult to sandwich the metal sheets from above and below in a closed cross-sectional structure. The indirect spot welding method is used in such cases.
When the indirect spot welding method is used in applications such as those described above, the overlapping metal sheets are pressed by an electrode from only one side, with the other side of the metal sheets being unsupported in midair. Therefore, unlike the direct spot welding method that sandwiches the metal sheets with electrodes on both sides, it is not possible to apply a high electrode force locally directly below the electrode. Furthermore, since the electrode sinks into the metal sheet during application of current, the conditions of contact between the electrode and the metal sheet and between the metal sheets change.
For these reasons, in conventional indirect spot welding, the current path between the overlapping metal sheets is unstable, making it difficult to reliably obtain an oval nugget formed after fusion between the metal sheets. In particular, when the overlapping metal sheets are pressed by an electrode from only one side, with the other side being unsupported in midair, and both edges of the metal sheets are restrained, then current flow between the metal sheets at a location other than the weld, i.e., shunt current, increases, making it even more difficult to reliably obtain an oval nugget.
As a welding electrode that can obtain a weld with predetermined strength and that can be applied to indirect spot welding, JP 2006-198676 A discloses an “electrode for resistance welding having a generally conical tip, comprising: a conical surface in which a tip angle of a cone is 120° to 165°; and a flat portion with a diameter of 1.5 mm to 3 mm in a central portion of a tip of the cone.”
With regard to a weld formed according to the disclosed technique, however, JP '676 discloses that “observation of metal structures of overlapping portions between the metal sheets 11 and 12 reveals events where, as compared to a conventional typical nugget, the metal of the overlapping portions between the metal sheets 11 and 12 includes many portions in which the metal is finely and partially fused and is recrystallized. This is the case where the metal sheets 11 and 12 are joined together by so-called diffusion bonding, and the metal sheets 11 and 12 may be joined together by events different from those of conventional typical nuggets” (paragraph [0038] of JP '676). In other words, a problem exists in that the weld obtained using the welding electrode disclosed in JP '676 is not necessarily an oval nugget formed after complete fusion, as in a nugget resulting from direct spot welding.
As described above, current control standards used by transport equipment manufacturers for spot welds often require that welds be oval nuggets. Therefore, even if a predetermined weld strength can be obtained for a weld, the weld will not satisfy the control standards if the nugget is not an oval nugget formed after complete fusion. Accordingly, there is a desire for an indirect spot welding method that can more reliably obtain an oval nugget even when current flow between metal sheets at a location other than the weld, i.e., shunt current, is large.
It could therefore be helpful to provide an indirect spot welding method that can more stably obtain an oval nugget formed after fusion between metal sheets even when current flow between the metal sheets at a location other than the weld, i.e., shunt current, is large.